1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a transmissive liquid crystal display device using a cholesteric liquid crystal color filter layer.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices with light, thin, and low power consumption characteristics are used in office automation equipment, video units and the like. Such LCDs typically use a liquid crystal (LC) interposed between upper and lower substrates with an optical anisotropy. Because the LC has thin and long LC molecules, the alignment direction of the LC molecules can be controlled by applying an electric field to the LC molecules. When the alignment direction of the LC molecules is properly adjusted, the LC is aligned and light is refracted along the alignment direction of the LC molecules to display images.
In general, LCD devices are divided into transmissive LCD devices and reflective LCD devices based upon whether the display device uses an internal or external light source.
A related art LCD device includes an array substrate, a color filter substrate, and a liquid crystal interposed between the array and color filter substrates. In general, voltages are applied to two electrodes which are formed on the array and color filter substrates, respectively, whereby an electric field generated between the two electrodes moves and arranges molecules of the liquid crystal. In order to display images in the LCD device, light should pass through the liquid crystal. Therefore, a backlight device is required to generate the light to pass through the liquid crystal.
A related art LCD device has an LCD panel and a backlight device. The incident light from the backlight is attenuated during the transmission so that the actual transmittance is only about 7%. A transmissive LCD device requires a high, initial brightness light source, and thus electrical power consumption by the backlight device increases. A relatively heavy battery is needed to supply sufficient power to the backlight of such a device, and the battery can not be used for a lengthy period of time.
In order to overcome the problems described above, a reflective LCD has been developed. Because the reflective LCD device uses ambient light instead of the backlight by using a reflective opaque material as a pixel electrode, the reflective LCD may be light and easy to carry. In addition, the power consumption of the reflective LCD device may be reduced so that the reflective LCD device can be used as an electric diary or a PDA (personal digital assistant).
However, the reflective LCD device is affected by its surroundings. For example, the brightness of ambient light in an office differs largely from that of the outdoors. Therefore, the reflective LCD device can not be used where the ambient light is weak or does not exist. Furthermore, the reflective LCD device has a problem of poor brightness because the ambient light passes through the color filter substrate and is reflected toward the color filter substrate by a reflector on the array substrate. Namely, because the ambient light passes through the color filter substrate twice, the reflective LCD device has a low light transmissivity and thus, poor brightness.
In order to overcome the above-mentioned problem, it is necessary to improve the transmittance of the color filter. To improve the transmittance, the color filter needs to have low color purity. However, a limitation is encountered when lowering the color purity because it is difficult to form a color filter thickness under a critical margin using a color resin. Accordingly, an LCD device having a layer for selectively reflecting and transmitting light is being researched and developed.
In general, liquid crystal molecules have a specific liquid crystal phase based on their structure and composition. The liquid crystal phase is affected by temperature and concentration. The most common liquid crystal is nematic liquid crystal in which the molecules of liquid crystal are oriented in parallel lines in one direction. The nematic liquid crystal has been extensively researched and developed and applied to various kinds of liquid crystal display devices.
Recently to improve the operating characteristics (such as brightness) of the transmissive LCD device, a cholesteric liquid crystal (CLC), which selectively transmits or reflects light with a specific color, has been studied and developed. The CLC usually has liquid crystal molecules whose axes are twisted or includes chiral stationary phase molecules and nematic liquid crystal molecules that are twisted by the chiral stationary phase molecules. In general, the nematic liquid crystal has a regular arrangement in parallel to one another, while the cholesteric liquid crystal has a multi-layered structure. The regular arrangement of nematic liquid crystal appears in each layer of the cholesteric liquid crystal.
Furthermore, the CLC has a helical shape and the pitch of the CLC is controllable. Therefore, the CLC color filter can selectively transmit or/and reflect the light. In other words, as is well known, all objects have an intrinsic wavelength, and the color that an observer recognizes is the wavelength of the light reflected from or transmitted through the object. The wavelength (λ) of the light reflected by the CLC can be represented by the following formula, which is a function of pitch and average refractive index of CLC: λ=n(avg)·pitch, where n(avg) is the average index of refraction. For example, when the average refractive index of CLC is 1.5 and the pitch is 430 nm, the wavelength of the reflected light is 645 nm and the reflective light becomes red. In this manner, the green color and the blue color also can be obtained by adjusting the pitch of the CLC.
In other words, the wavelength range of visible light is about 400 nm to 700 nm. The visible light region can be broadly divided into red, green, and blue regions. The wavelength of the red visible light region is about 660 nm, that of green is about 530 nm, and that of blue is about 470 nm. Due to the pitch of the cholesteric liquid crystal, the CLC color filter can selectively transmit or reflect light having the intrinsic wavelength of the color corresponding to each pixel thereby clearly displaying the colors of red (R), green (G) and blue (B) with a high purity. In order to implement a precise color, a plurality of the CLC color filters can be arranged, to display the full color more clearly than a color filter of the related art. The cholesteric liquid crystal (CLC) color filter will be referred to as CCF herein after.
FIG. 1 shows a conceptual arrangement of the cholesteric liquid crystal color filters (CCFs) according to a related art. When the CCFs are used for the color filter substrate of the LCD device, red (R), green (G) and blue (B) sub-pixels constitute one pixel. Therefore, the R, G and B sub-pixels are in the ratio of 1:1:1 within one pixel. Namely, the R, G and B CCFs have the same size when they are applied to the related art color filter substrate. Furthermore, two of the R, G and B CCFs are accumulated and formed in each sub-pixel to produce the other one of the red, green and blue colors.
In FIG. 1, a first CCF layer 12 and a second CCF layer 14 are formed in series over a substrate (not shown) where the plurality of R, G and B sub-pixels are defined. The first CCF layer 12 has red (R), green (G) and blue (B) CCFs in an alternate order, and the second CCF layer 14 also alternately has red (R), green (G) and blue (B) CCFs. In the red (R) sub-pixel that produces the red color, the G and B CCFs are disposed. Additionally, the R and G CCFs are disposed in the blue (B) sub-pixel, and the R and B CCFs are disposed in the green (G) sub-pixel.
In the R sub-pixel, the green (G) CCF of the first CCF layer 12 selectively reflects green-colored light and transmits red- and blue-colored light. Thereafter, the blue-colored light is reflected by the blue (B) CCF of the second CCF layer 14 in the R sub-pixel. As a result, only the red-colored light transmits the blue (B) CCF of the second CCF layer 12, and then the R sub-pixel can produce the red color. In this manner, the other blue and green colors can be produced in the B and G sub-pixels, respectively.
According to the related art, one pixel has the R, G and B sub-pixels each having double-layered CCF layers. Furthermore, the sizes of the R, G and B CCFs are the same with the ratio of 1:1:1. However, such ratio of 1:1:1 has some disadvantages.
FIG. 2 shows CLC color filters of the transmissive LCD device after pixelating the R, G and B CCFs in the ratio of 1:1:1 according to a related art. As shown, red, green and blue sub-pixels SP are disposed in an alternate order. However, a color mixture occurs in borders B among the R, G and B CCFs. The reason of the color mixture may be caused by the misalignment of R, G and B CCFs, or by the light leakage during the formation of the CCFs. When forming the R, G and B CCFs, a mask is disposed over the cholesteric liquid crystal (CLC) layer and then the light exposure is performed on the CLC layer. At this time, the exposure light diffuses and affects the other portions for CCFs, whereby abnormally exposed portions exist in the borders B among the red, green and blue CCFs.
Moreover, there are some other problems when forming the R, G and B CCFs in the ratio of 1:1:1. The cholesteric liquid crystal (CLC) layer becomes the R, G and B CCFs depending on how much and how long it is exposed to the light. The primary CLC layer is formed with uniform thickness over the substrate, but the formed R, G and B CCFs have the different thickness due to the amount of the light for exposure. In general, the G CCF is thicker than the B CCF by 0.25 micrometers, and the R CCF is thicker than the G CCF by 0.25 micrometers. Namely, the CCF layer including the R, G and B CCFs may have the maximum thickness difference of 0.5 micrometers in the pixel. Such thickness difference is shown in the graph of FIG. 3. When the steps (i.e., the thickness differences) exist in the CCF layer, they lead to the different cell gaps in the pixel between the color filter substrate and the array substrate, thereby bringing about a result of retardation difference. Referring back to FIG. 1, the second CCF layer 14 may planarize the surface of the first CCF layer 12, but second CCF layer 14 also has the thickness difference (the step) among the R, G and B CCFs because it is also fabricated by the light exposure.